|DayThree: Igneous Processes in the Moruya Batholith, Bingie Bingie Point, NSW Australia.
- Describe and recognise minerals in hand specimen. Minerals will inlcude: quartz, plagioclase, potash feldspar, amphibole, garnet, biotite, and muscovite.
- Describe and name quartz-saturated magmatic rocks.
- Measure and report on a map planar and linear features.
- Produce a 1:1000 scale geological map of Bingie Bingie Point (one per group) showing the distribution of different rock types, orientation of the fabrics and field relationships of all igneous rocks exposed.
- Learn to interpret microstructures and textures in terms of igneous processes including pluton emplacement, magma migling and mixing, and the generation of new continental crust.
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The mapping of granitoid complexes like the Moruya batholith provides information of the physical and chemical processes involved in the evolution of continental crust. Bingie Bingie point provides some spectacular exposures of igneous rocks that show a wide range of microstructures, textures and intrusive relationships between rocks of variable composition. These features allow us to look at how magma batches interacted during the construction of the Moruya batholith.
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Each participant must turn in a finished map and a short 2-3 page report complete with outcrop sketches by the end of day 3. You will be assessed on 3 aspects:
- the quality of your description and observation,
- the quality of your map,
- the depth of your report and pertinence of your interpretation of the various microstructures you have seen.
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QAP diagram: The classification of plutonic rocks
The classification for quartz-saturated magmatic rock with less that 10% of mafic minerals (amphibole, mica) proposed by Streickensen is based of the proportion of felsic minerals: Quartz, Alkali-feldspar (including orthoclase, microcline, perthite, anorthoclase, sanidite, and albite An0 to An5), and Plagioclase (An5 to An100).
In red are fine-grained (aphanitic) volcanic equivalent to coarse-grained (phaneritic) plutonic rocks. Gabbro and diorite differ only by the composition of their plagioclase (An>50 for gabbro, An<50 for diorite).
Magma that solidifies well below the surface forms plutonic intrusions. Extrusive volcanic rocks form from magma that solidifies on the surface. Crystal size is largely related to the length of time the rock takes to solidify. Hence plutonic intrusion are characterized by coarse grained aggregates with interlocking crystalline phaneritic texture. In contrast volcanic rocks form by rapid cooling are characterised by fine-grained (aphanitic) to micro-crystalline (glassy) textures.
Feldspar: it is the most important group of rock-forming minerals that make up to 60% of the Earth's crust. Feldpars are essential constituents of most igneous rocks and are therefore used the the classification of the igneous rocks. Felspars are composed of a network of SiO4 tetrahedron in which some of the Si4+ are replaced by Al3+. The building blocks of feldspar are either (AlSi3O8)- or (Al2Si2O8)2-. The electrical balance is restored by ions such as Na+, K+, and Ca2+. Therefore the composition of most feldspar can be represented on a ternary diagram with KAlSi3O8 (Potash feldspar, Or), NaAlSi3O8 (Albite, Ab), and CaAl2Si2O8 (Anorthite, An). Feldspar with compositions ranging from Or to Ab are called Alkali Feldspar. Feldspar with compositions ranging between Ab and An are called Plagioclase.
In hand speciment, orthoclase (also called K-Feldspar of potash-feldspar) and sanidine often have a well defined crystal shape and often exhibit a simple twinning. The twinning plane is parallel to the long axis of the crystal. They are pinkish porcelain like mineral that has a rectangular shape. They form at an early stage during the crystallisation on granitic magmas therefore often developing crystal faces. Plagioclases in contrast tends to define a milky-white to pale green mineral and, because they crystallise at a later stage, often form interconnected groundmass whitout showing distinctive crystals. The picture of the tonalite below illustrate this point.
Quartz (SiO2) has no cleavage which makes quartz resistent to erosion. It is a glassy, irregularly shaped grains typically lacking well-developed crystal faces. In igneous rocks it is the last mineral to form and therefore defines crystallized groundmass often transluscent, pale to dark grey in color. It is stronger that the feldspar which makes it able to scratch the metal of geological hammer.
Amphibole: They general formula is X2Y5(Si, Al)8O22(OH)2. With X=Mg, Fe, Ca or Na, and Y=Mg, Fe2+, Al, or Fe3+. Dark green in colour, they form prismatic crystals with a characteristic double cleavage intersecting at angles of 56 to 124 degres.
Micas: In igneous rock they typically form tiny shiny flakes. Its perfect one-directional cleavage, which permits breakage into thin elastic flakes, is readilly recognizable. Biotites and muscovites are shiny black and white micas respectively. The composition of biotite incorporates the following building blocks: SiO4 , AlSiO3 , FeO, MgO, K2O, and H2O. The composition of muscovite incorporate also CaO, and Na2O.
Garnet: Form well crystallized crystal of spherical shape. Garnets are glassy crysal with no cleavage and have a characteristic pink to dark-red color. Their composition is made of: SiO4 , Al2O3 , FeO, and MgO.
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Descriptors of Igneous Rocks
TEXTURES: Igneous rocks originates from the partial melting of the crust (felsic magma) and the mantle (basaltic magma). The texture of igneous rock characterizes the size, the shape and the arrangement of mineral. It provides important insight into the cooling history of the magma.
Glassy texture: The hand specimen displays concoidal fractures with sharp edges typical of broken glass. No distinct grains are visible even with an hand-lenses. Glassy texture is produced by very rapid cooling (quenching), ions do not have time to organize themselves in an orderly crystalline structure.
Aphanitic texture: It consists of a very fined grained groundmass with mineral too small to be seen without a hand-lenses. This groundmass may includes a few grains large enough to be seen. Aphanitic texture result from rapid cooling. A feature in many aphanitic texture is the presence of numerous small spherical to ellipsoidal cavities called vesicles. They are produced by trapped gas bubbles formed during the decompression of the magma. Glassy and aphanitic textures are produced by volcanic eruption and magma crystallising very close to the surface.
Phaneritic texture: It consists of grains large enough to be seen in hand specimen. All grains are about the same size (equigranular) and interlock to form a compact mosaic. The larger the crystals the slower the cooling rate, which suggest that the magma crystallized at depth (>5km).
Porphyritic texture: Often igneous rocks have grains of two distinct sizes. The larger, well-formed crystals are referred to as phenocrysts, and the smaller crystals constitute the matrix also called the groundmass. This texture is called porphyritic, it suggests two stages of cooling: an initial stage of slow cooling, during which large grains develop, is followed by a period of more rapid cooling, during which the groundmass forms. Depending of the texture of the groundmass one can discriminate between porphyritic-glassy, porphyritic-aphanitic, and porpheritic-phaneritic textures.
GEOMETRY OF INTRUSIONS: Igneous rocks form when a magma cools below the surface. The shape of igneous rock depends on the mechanisms that control their emplacement. Magma rises to the surface because of their density is lower than that of the surrounding rocks. The mechanism that control the migration of magma through the crust are diverse, however the two most important mechanisms are diapir and dike.
Batholiths: they are the largest igneous rock bodies in the Earth's crust. They are generally of granitic composition forming elongated bodies covering thousand of square kilometers. A batholith can result from a single and multiple intrusions and can develop from a mixture of mafic and felsic magmas.
Stocks: A stock is an intrusive body with an outcrop area of less than 10km2. They may form granitic protrusion rising from a batholith.
Lacocoliths: They are lens-shaped masses of igneous rock injected between layers of the surrounding rock. They tend to deform the overlying strata in a dome-like structure with a flat floor and an arched roof (mushroom head). laccoliths can be several kilometers in diameter and thousands of meters thick.
Dikes: Dikes are narrow tabular bodies. They form when a magma squeezes into fractures and cools. The width of adike can range from a fraction of a centimeter to hundreds of meters. The Great Dike of Zimbabwe is 600km long and has an average width of 10km. Discordant and concordant dikes are oblique and parallel respectively to the main fabric (bedding, cleavage, folation etc) of the rock they intrude.
Sills: Rising magma follows the path of least resistance. When this path includes an horizontal zone of weakness (bedding plane, cleavage, foliation) the magma may be injected horizontally to form a sill: a tabular body concordant with the layering. Sills range form a few centimeters to hundred of meters thick, and ccan extend laterally for several kilometers. Sills can form as local offshoots from dikes, or they can be connected directly to a stock or batholith. Sills and dikes may contain inclusions, blocks and minerals of the surrounding rocks.
MAGMA INTERACTION: In the field, felsic and mafic magma can coexist in the same body. Mafic and felsic magmas do not mix together into a magma of intermediate composition. In contrast they form coherent entities (enclaves, dikes, sills). Their respective mineralogical composition is not altered exepted at their contacts. Mafic magmas, because they form at higher temperature (~1200C), tend to be less viscous and more mobile and the felsic magma they intrude, which is at a much lower temperature (900-1000C). Mafic magmas form dikes in which they freeze, whereas enhanced melting occurs in the felsic magma at the contact with mafic dikes. If the cooling rate of the felsic magma is slow and if the felsic magma is mobile, mafic dikes can be broken-up into small enclaves of ellipsoidal shape or angular blocks when the mafic magma is too viscous. Mafic enclaves may align themselves to define a magmatic fabric. Alternatively, mafic magmas can intrude and rise through the felsic magma as bubble to form mafic enclaves. The dispersion of mafic enclaves into a felsic magma is called magma mingling.
Mafic enclaves should not be confused with xenoliths that represent exotic blocks of the surrounding rocks.
Rocks at Bingie Bingie Point
Tonalite, with a phaneritic texture.
Diorite-Tonalite layered rock
Mafic dikes with inclusions aggregating in the central plane.
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Magmatic Fabric in tonalite. The fabric is indicated by the prefered orientation of the long axis of the mafic enclave.
As you look closer you may find hard to pick up this fabric. On that picture, try to focus your attention on discrete change in colour. This change in colour are related to minute changes in mineralogical composition that may indicate the presence of a magmatic banding. Look also for small aggregates of mafic minerals (schlieren) that can be oriented in a direction parallel to the fabric. Look also for blocky mineral such as plagioclase an amphibole you may notice that they are statistically oriented along a common direction.
In this picture a mafic dike cut through the tonalite. The hammer is oriented parallel to the magmatic fabric marked by the alignement of mafic enclaves.
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Lobate contact between tonalite (below) and diorite (above).
Mafic enclaves in tonalite. Note the leucocratic hallo around the mafic enclave. This hallo is the result of enhanced melting of the tonalite heated by the mafic inclusion. This occured at an early stage during which a fair temperature contrast existed between the tonalitic magma and the mafic magma.
Along with well rounded enclave, mafic inclusions occur as angular blocks floating in the tonalite. These angular blocks indicate that the gabbo was fully crystallised and therefre reacted brittely to local deviatoric stresses imposed by the tonalitic magma.
The picture below shows a sedimentary xenoliths in the tonalite.
Locally mafic and felsic magmas are intimately associated in a migmatite. Both magmas form layers that can locally be folded. The these folds are developed during the flow of the magma as indicated by the absence of solid state axial planar fabric.
Along with migmatite-type structures one can observe felsic veins breaking the diorite into angular blocks. The difference between this picture and the one before is the background temperature. The temperature in the migmatites implies that both the felsic and mafic magma are well above their respective solidus. In contrast the picture below suggests that the temperature is such that the mafic magma is fully crystallised whereas the felsic magma is still well above its solidus.
Veins of tonalitic melt isolating angular clast of diorite.
Extensional fractures filled with granitic melts.
The tonalite melt evolves toward a granitic composition (K-feldspar in pink).
This picture show a mafic dike intrusive in the tonalite. This dike is full of inclusions including tonalitic clasts. The dike seems to be affected by brittle faults however there is no deformation in the dike and its host rock. In fact, these apparent offsets run parallel to the magmatic foliation in the tonalite. Those little breaks can be interpreted as analogous to transform faults that affect mid-oceanic ridges. Here they devolp parallel to a plane of weakness which is the tonalite correspond to the magmatic foliation.
As the dikes growth wide these steps remains.
One generation of dike at Bingie Bingie is full of inclusions...
...including blocks of tonalite and other plutonic rocks...
...and metamorphic minerals such as garnets...
These loaded dikes are intrusive in the tonalite and the gabbro. They also cut through an early generation of felsic dikes.
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The group at work...
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